Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, such devices are also utilized for sensor applications (e.g., chemical vapor detection). Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the acoustic wave sensor.
Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure. An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. Bulk acoustic wave device are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of masks.
Based on the foregoing, it can be appreciated that acoustic wave devices, such as a surface acoustic wave resonator (SAW-R), surface acoustic wave delay line (SAW-DL), surface transverse wave (STW), bulk acoustic wave (BAW), can be utilized in various sensing measurement applications. One of the primary differences between an acoustic wave sensor and a conventional sensor is that an acoustic wave sensor can store energy mechanically. Once such a sensor is supplied with a certain amount of energy (e.g., through RF), the sensor can operate for a time without any active part (e.g., without a power supply or oscillator). This feature makes it possible to implement an acoustic wave sensor in an RF powered passive and wireless sensing application.
One promising application for micro-sensors involves oil quality monitoring, particularly in the area of deep fry oil cooking implements. Restaurants and food industries currently rely on cooking oil color and foaminess as indicators of oil quality. Researchers have indicated however, that these factors are not sufficient to determine the quality of cooking oil. It has been estimated that twenty-five percent of cooking oil can be converted to free fatty acids and other unhealthy compounds before the color of the cooking oil actually changes. This means that fried foods may be cooked unknowingly in oil of an unacceptable quality, which can ultimately lead to immediate and long term trouble for the consumer. For example, if the oil is rancid, the consumer may become sick. Alternatively, long term health problems associated with consuming unhealthy compounds from poor quality oil may also result.
Chemical changes that take place in cooking oil can make food cooked in such oil harmful to the consuming public. During the heating process, for example, hundreds of reactions take place in the cooking oil. Some of the products of these reactions escape in the form of gases, while others remain in the cooking oil. Remaining compounds include decomposition products and free fatty acids—the building blocks of oil, which are ultimately toxic to the human body. Such fatty acids, for example, cause the upset-stomach feeling that a typical consumer experiences following digestion of fried food.
What is needed to address such problems is the implementation of an oil quality sensor, particularly one which can be monitored wirelessly. It is believed that the embodiments disclosed herein address these long felt needs.